In sheet glass grinding, so-called two-sided grinding machines are used, which comprise a succession of grinding wheels for grinding the opposite lateral edges of the sheet; and two corner bevelling assemblies, downstream from the grinding wheels in the travelling direction of the sheet, for grinding the front and rear corners of the sheet.
Each corner bevelling assembly comprises a vertical-axis grinding wheel; a first powered guide-slide assembly for moving the grinding wheel in a longitudinal direction parallel to the travelling direction of the sheet; and a second powered guide-slide assembly for moving the grinding wheel to and from a forward work position in a transverse direction perpendicular to the longitudinal direction.
To grind the corners of the sheet, the sheet is fed longitudinally towards the corner bevelling assembly at a substantially given speed; as the sheet moves forward, the grinding wheel is first moved in the transverse direction towards the sheet and into the forward work position by the second guide-slide assembly; and, once the position of the sheet is determined, the first guide-slide assembly eases the grinding wheel towards the sheet in the longitudinal direction, to minimize impact between the sheet and the grinding wheel waiting in the forward work position.
Though widely used, known corner bevelling assemblies have the major drawback of being difficult to control, or at least accurately enough to prevent the sheet from slamming directly against the grinding wheel, thus resulting in chipping or breakage of the sheet, which is therefore eventually rejected.
The sheet slamming against the grinding wheel may even damage the grinding wheel itself, so that, unless the wheel is sharpened frequently, grinding quality becomes inconsistent.
The above drawback is caused by various factors, foremost of which is failure of the first guide-slide assembly actuator—be it pneumatic or electric—to accurately control the movement of the grinding wheel with respect to the sheet, to ensure steady, smooth contact between the sheet and the grinding wheel, and consistent bevel quality.
Moreover, in the event of wear of the sheet conveyors and/or errors in detecting the position of the sheet along its route, it is practically impossible to determine the exact position of the sheet. As a result, the sheet may slam into the grinding wheel, as stated above, but may often even fail to contact the wheel at all, which on the one hand saves the grinding wheel, but on the other invariably results in dimensional errors in grinding the corners.